function main() {
    var canvas = document.getElementById('webgl');
    if(!canvas){
        console.log('your brower do not support Canvas');
        return;
    }
    // get the draw context

    var ctx = canvas.getContext('2d');
    // draw rect
    ctx.fillStyle = 'rgba(0,0,255,1.0)';
    ctx.fillRect(120,10,150,150);

}